Sensing the location of a magnetic material or magnet by using magnetoelectric conversion elements such as Hall elements or magnetic resistance elements has been a widely practiced method, for example, in sensing the phase of a DC brushless motor. However, even a GaAs Hall element, which usually has a small temperature coefficient, has a temperature coefficient of -0.06%/.degree.C. during constant-current drive. Therefore, the displacement could not be accurately sensed in a wide temperature range.
In order to overcome this problem:
1) Japanese Patent No. 1094798 (Japanese Laid-Open Patent Application No. 38073/1981) provides a compound magnetoelectric conversion means which is composed of Hall elements and a temperature sensing element all closely placed on a base material in the same pattern, and which is thereby capable of performing excellent temperature compensation even for local temperature variations in the element itself. PA1 2) In Japanese Laid-Open Patent Application No. 171879/1983, the variations of output voltage caused by the temperature variations of the Hall elements are controlled by forming compensation resistors in series on a base member. PA1 3) Japanese Laid-Open Patent Application No. 248010/1989 is aimed at obtaining a high precision throttle sensor that is independent of temperature variations, with the polarities of the temperature coefficients of the magnet and the Hall elements opposite to each other.
Although the compound magnetoelectric conversion elements described in 1) and 2) are excellent in temperature compensation, the production of these compound magnetoelectric conversion elements is difficult and costly.
Furthermore, these compound magnetoelectric conversion elements are not generally used, and therefore hard to come by.
In a displacement sensor comprising a permanent magnet and megnetoelectric conversion elements as described in 1) and 2), the surface magnetic flux density of the permanent magnet itself has temperature characteristics, and therefore, compensation only for the temperature characteristics of the magnetoelectric conversion elements is not enough to compensate for the temperature characteristics of the whole displacement sensor.
In the case of 3), the problem is the difficulty in adjusting the temperature coefficients of the magnet and the Hall elements so that they are opposed to each other. Though Si Hall ICs are used in place of Hall elements to provide a switching sensor, the electron mobility of Si is so small that the swicthing sensor is not suitable for a high precision sensor. Further, Si Hall ICs have a large offset voltage, which causes variations in temperature characteristics.
Thus, the displacement sensor the prior art is not capable of performing accurate sensing due to temperature variations and is not suitable for the use in a wide temperature range.
Accordingly, the present invention is basically aimed at providing a displacement sensor that is capable of accurately sensing the displacement in a wide temperature range by compensating for the temperature coefficients of a permanent magnet and magnetoelectric conversion elements. Even when the displacement sensor is used in a place where the temperature varies greatly, the displacement is sensed with high precision by the displacement sensor.